From the first successful p-type doping of GaN with Mg, many devices have been constructed with the objective of producing commercially viable lasers in the ultraviolet and blue range of the optical spectrum. However, although limited lasing has been achieved, there remain many problems to be overcome. One of the main problems is that the operation voltages and the threshold currents are too large. The main reason for this is that even the use of contacts consisting of metals of the highest workfunctions does not produce perfect ohmic contacts to p-type GaN. Thus, metals such as Ni, are used and the resultant non-perfect ohmic contact is accepted as an unwanted but inherent feature of the GaN based devices. Many have attempted to employ various temperature annealing methods to lower the contact resistance of the used meal contacts without any substantial success (see H. Ishikawa, S. Kobayashi Y. Koide, S. Yamasaki, S. Nagai, J. Umezali, M. Koike, and M. Murakami; "Effects of Surface Treatments and Metal Work Functions on Electrical Properties at p-GaN/Metal Interfaces," J. Appl. Phys., 81 (1997) p1315), this is even the case for contacts to n-type GaN, where the problems are an order of magnitude lower than that of the subsequently grown p-type regions (see F. Ren, S. J. Pearton, S. Donovan, C. R. Abernathy and M. W. Cole; "Ohmic Contacts on Binary and Ternary Nitrides," Electrochemical Society Proceedings, Vol. 96-11, 1996, P122).
The chemical treatment of the GaN surfaces prior to metal evaporation has also been reported on (see for example L. L. Smith, S. W. King, R. J. Nemanichand, R. F. Davis; "Cleaning of GaN Surfaces," J. Electronic Materials, Vol. 25(1996) p805) and most people skilled in the art agree that the use of hydrofluoric acid (HF) based etches produces the best surface for subsequent contacts. Thus, most of the work on the production of low resistivity contacts to p-type and n-type GaN are now mainly along the lines of trying to boost the dopant densities in the GaN contact layers and/or the use of intermediate band materials, such as InGaN based materials, to reduce the effective barrier seen by the carriers trying to enter the GaN contact regions.